Ovarian hormones control the development and function of reproductive organs in females. In the uterus, estrogen binding to the cytoplasmic estrogen receptor and subsequent nuclear translocation of the complex are prerequisities for biological action. The reactions which allow transformation of the unactivated cytoplasmic estrogen receptor into an activated complex capable of nuclear binding are not understood. We will examine this receptor activation reactions using a purified preparation of calf uterine estrogen receptor. Purification will be performed under conditions which yield the native unactivated receptor estrogen complex. With the aid of chemical probes we will examine and compare the physiochemical properties of the unactivated and activated estrogen receptor. Comparative analysis of the activation reaction will be performed with receptor antiestrogen complexes.